The invention relates to a particular construction of Savonius rotor blade, a particular Savonius vertical axis wind turbine rotor, and a watercraft using a particular Savonius vertical axis wind turbine to power a propulsion device (such as a substantially horizontal axis propeller). The Savonius rotor blade, and rotor, according to the present invention have numerous advantages over prior art Savonius blades and rotors. In particular, because of the unique construction of the blades according to the present invention, a three bladed Savonius rotor is provided which can be expected to operate much more smoothly and effectively than conventional two bladed Savonius rotors, and with a higher maximum power coefficient (Cp) than known three bladed Savonius rotors.
In the following specification and claims the following terms have the indicated meanings:
“Cp” or “maximum power coefficient” means (as it normally does): Turbine torque times turbine rotational speed divided by freestream dynamic pressure times freestream velocity times the turbine swept area; or proportional to maximum power divided by swept area [that is Cp=P/[½ A ρ v3] where P=power, A=swept area, ρ=the density of air (about 1.2 kg/m3 at sea level and 70 degrees F.), and v=wind velocity].
“Tip Speed Ratio” or “TSR” means (as it normally does): blade tip speed divided by wind speed. For drag wind turbines this ratio is always less than one.
“Curvature” of a blade means: The ratio of the radius of the blade to the depth. The smaller the ratio, the more pronounced the curvature.
“Skew factor” of a blade means: The maximum curvature depth location along the radius of a blade from the axis of rotation. The larger the skew factor, the closer the maximum curvature depth is to the free end of the blade.
“Aspect ratio” means (as it normally does): The ratio of the length (height) of a rotor (or blade) to its diameter.
“Effective gear ratio” means: The rpm ratio between a driving and a driven component, whether gears or some other mechanical structure (such as chains and sprockets, pulleys and belts, cones and belts, etc.) are used to provide the operative connection between the driving and driven components.
“Operatively” or “operative” means (as it normally does): Any connection or engagement that allows the components connected or engaged to function as designed.
Although from the time of filing his first patent application in 1924 (see canceled FIG. 6 of GB published specification 244,414) Sigurd Savonius—the inventor of the Savonius rotor—contemplated a three bladed version as well as two bladed versions, more than eighty years later there are few [e. g. see Environmental Building News, Vol. 13, #4, April, 2004, p. 7, “Solar and Wind-Powered Outdoor Lighting from MoonCell”] commercial versions of the three bladed version. Perhaps because extensive wind tunnel testing by Sandia Laboratories in 1977 [Blackwell et al, “Wind Tunnel Performance Data For Two And Three-Bucket Savonius Rotors”, SAND76-0131, July, 1977] concluded “The maximum power coefficient of the two-bucket configuration is approximately 1.5 times that for the three-bucket configuration”, there has been almost no attempt to optimize a three bladed Savonius rotor. Conversely, there has been a great deal of work done on optimizing two bladed configurations [for example see Khan, “Model And Prototype Performance Characteristics Of Savonius Rotor Windmill”, Wind Engineering, Vol. 2, No. 2, 1978, pp. 75-85].
It has been found according to the present invention, however, that if a three bladed configuration of a Savonius rotor is optimized, the three bladed version can have advantages over and at least be competitive with two bladed versions. In addition to operating more smoothly, it can be just as easy or easier to manufacture; can have a Cp as great as, or greater than, two bladed versions with the same aspect ratio; and self-starts more easily. A critical factor in the optimization of a three bladed Savonius rotor is the skew factor, something not even recognized as a result-effective variable for three bladed Savonius rotors in the prior art. It has been found that a high skew factor (e. g. at least about 0.6, preferably over about 0.7, and most preferably about 0.75-0.85), along with significant curvature, results in a rotor with a Cp about 2-5 times greater than those with similar curvatures but lower skew factors, e. g. 0.25 or 0.5 (about 0.5 being the common skew factor for three bladed Savonius rotors).
According to one aspect of the present invention there is provided a Savonius vertical axis wind turbine (“VAWT”) rotor comprising: three blades operatively connected together to define a vertical axis wind turbine rotor; the blades having a curvature of greater than about 7:1, and a skew factor of greater than about 0.6. The rotor preferably also comprises at least one substantially vertical shaft, with the blades operatively connected to the shaft. The rotor preferably has an aspect ratio of at least about 0.8:1, more preferably at least about 2:1 (e. g about 3:1) Also, preferably the blades have a skew factor of about 0.7-0.9, e. g. about 0.75-0.85 or about 0.75-0.8, and a curvature of about 2:1 to 5.5:1, e. g. about 2.5:1 to 5:1. Such a rotor may be expected to have a Cp significantly greater than otherwise similar rotors with lower skew factors. That is the Cp of a rotor according to the invention can be expected to be at least about twice that of an otherwise identical rotor with a skew factor of 0.5 or below
Instead of the conventional construction of a Savonius rotor, which includes at least top and bottom discs to which vanes are attached to form the blades, typically with no central shaft between the discs, preferably the blades of the rotor according to the invention comprise a plurality of substantially vertically aligned spokes axially spaced along the at least one shaft, each spoke comprising three generally radially extending ribs; and vanes of sheet material operatively connected to the ribs. Preferably the vanes are substantially straight vertically, substantially devoid of twist, although in some circumstances a slight twist can be provided. While a wide variety of materials may be used to construct the Savonius rotor, preferably the ribs are made of substantially rigid (e. g. plates or bars) aluminum, titanium, carbon fiber, pvc, or steel alloy, and the vanes are made of sheet material of aluminum, titanium, carbon fiber, steel alloy, Pentex (modified low stretch polyester), polycarbonate (e. g. Lexan®), or other plastic having substantially the same strength, structural integrity, and durability properties as polycarbonate.
While plural shaft versions of the Savonius rotor according to the invention—such as shown in co-pending application Ser. No. 10/854,280 filed May 27, 2004 (the disclosure of which is hereby incorporated by reference herein)—and other versions with spillover are within the scope of the invention, multiple shafts and significant spillover are not necessary to achieve a high Cp when practicing the invention. That is, the Savonius rotor according to the invention may comprise a single shaft, with each spoke comprising a hub surrounding the shaft and operatively connected thereto to substantially preclude movement with respect to the shaft, the ribs extending generally radially outwardly from the hub.
The Savonius rotor of the invention may be used to power a boat (e. g. by driving a propeller), such as disclosed in co-pending application Ser. No. 10/443,954 filed May 23, 2003, power a generator to generate electricity (as disclosed in U.S. Pat. No. 6,172,429), power a pump to pump water or other liquids, or be used in combination with virtually any other conventional driven element. According to another aspect of the invention, the Savonius rotor is in combination with a driven element and a drive operatively connects the driven element to the rotor; the drive automatically increasing the effective gear ratio directly in response to an increase in the speed of rotation of the rotor. As one example, the drive may comprise a first sprocket operatively connected to the at least one shaft, and different size smaller at least second and third sprockets operatively connected to the driven element with a chain operatively connecting the first sprocket and one of the at least second or third sprockets; and a transmission comprising a centrifugal force responsive derailleur for automatically shifting the chain between the second and third sprockets. In this way as the speed of rotation of the rotor increases, so too does the rpm of the driven element so that start-up of the rotor is not hindered yet a high rpm of the driven element may be obtained.
According to another aspect of the present invention, a blade per se for a Savonius turbine rotor is provided. The blade comprises a plurality of substantially rigid ribs spaced from each other along a first axis and substantially in alignment with each other along that axis; and a vane of sheet material extending between the ribs and operatively connected thereto. The blade has a curvature of greater than about 7:1, and a skew factor of greater than about 0.7, e. g. a skew factor of about 0.75-0.85 and a curvature of about 2:1 to 5.5:1. Preferably the blade also has an aspect ratio of at least about 4:1 (about twice the skew factor of a rotor constructed therefrom).
According to yet another aspect of the present invention there is provided a wind powered boat comprising: a plurality of hulls (e. g. the boat is a catamaran or trimaran); a propulsion mechanism (such as a substantially horizontal axis propeller) operatively connected to at least one of the hulls and between two of the hulls; and a Savonius vertical axis wind turbine rotor having an aspect ratio of at least about 2:1, and comprising: at least one substantially vertical shaft; three blades operatively connected to the shaft; and the blades having a curvature of greater than about 6:1 (e. g. about 2:1 to 5:1), and a skew factor of greater than about 0.65 (and preferably about 0.75 or greater). The rotor is operatively mounted to at least one of the hulls and operatively connected to the propulsion mechanism.
It is the primary object of the present invention to provide an effective Savonius rotor having a wide variety of uses and used in a wide variety of manners while operating smoothly with a high Cp. This and other objects of the invention will become clear from a detailed description of the invention, and from the appended claims.